Carbon fiber produced nowadays include PAN- and rayon-based carbon fiber produced from polyacrylonitrile (PAN) and rayon as starting materials and pitch-based carbon fiber produced from pitches as starting materials. PAN-based carbon fiber is mostly of a high strength-type. On the other hand, pitch-based carbon fiber includes anisotropic carbon fiber and isotropic carbon fiber, of which the anisotropic carbon fiber has a high specific modulus and a high thermal conductivity because of a high crystal perfection and a high orientation of hexagonal net plane in the fiber axis direction, and is used in the fields of sports, leisures and aviation and space technology.
On the other hand, pitch-based isotropic carbon fiber is relatively inexpensive because of inexpensive starting materials and a production process advantageous for mass production, and is widely used in view of properties, such as lightness, chemical resistance, heat resistance, lubricity and electroconductivity.
Carbon fiber is used in various forms, such as filament, sliver, spun yarn, fabric, chops, milled fiber, mats, and prepreg, and its calcination temperature and degree of graphitization may be varied for use thereof. Among these, a carbon fiber woven fabric is used as components of insulating materials, sliding materials and electroconductive materials, and is required to show an affinity with polymeric materials, so that the control of thickness of and voids in the fabric is important.
In recent years, there have been made some proposals of using carbon fiber woven fabric as materials in the field of electronics, such as a gas diffuser in a solid polymer electrolyte fuel cell (e.g., Patent documents 1 and 2 shown below).
Principal functions of a gas diffuser in a solid polymer electrolyte fuel cell are supply of a reaction gas to a catalyst layer and collection of electricity. Accordingly, gas diffusivity and electroconductivity are the most important properties, but in addition thereto, flexibility, high tensile strength, etc, are required (Patent document 1 below).
As for the electroconductivity, a high electroconductivity can be attained through a heat treatment at a high temperature of 2000° C. or higher to provide an increased graphitization degree.
On the other hand, the gas diffusivity is determined by an aperture rate (a porosity ratio) of the fabric, but too coarse a porous material causes a problem in electricity collection due to poor contact with the catalyst layer. In the case of using a carbon fiber fabric as a gas diffuser, it has been disclosed that a spun yarn fabric is preferable than a filament fabric wherein unit filaments are liable to be aligned to provide a high density (Patent document 2 below). In view of supply of gas to the catalyst layer, the reaction gas has to reach the catalyst layer by diffusion through a distance corresponding to the thickness of the gas diffuser, so that too large a thickness of the gas diffuser provides a cause of a lower performance. Accordingly, it is necessary to appropriately control the thickness of the carbon fiber fabric as a gas diffuser.
For the above reasons, it may be concluded that a spun yarn fabric having an appropriate thickness and a thermal history of 2000° C. or higher is preferred as a gas diffuser. Such a spun yarn fabric may be obtained through a process of weaving a flame-resistant fiber or a carbon fiber to provide a fabric and heat-treating the fabric at a temperature of 2000° C. or higher, or a process of weaving a spun yarn heat treated at 2000° C. or higher. Fiber causes heat-shrinkage on heat-treating, so that heat-shrinkage of an insufficiently carbonized fabric is not desirable because the heat-treatment results in strained fiber.
As spun yarn, there are known spun yarn of PAN-based flame-resistant fiber and pitch-based spun yarn. The spun yarn of PAN-based flame-resistant fiber comprises yarn of a relatively small diameter, is strong and can be woven, but it causes a remarkable lowering of strength when heat-treated at 2000° C. and the weaving thereof becomes difficult. Accordingly, an objective fabric cannot be obtained without resorting to a process of weaving such a flame-resistant fiber and heat-treating it at 2000° C. However, the resultant fabric is accompanied with a serious defect of a lower strength because of a strain of the fiber and a lowering in strength due to the heat treatment. For this reason, in order to be used as a gas diffuser, the carbon fiber fabric has to be subjected to supplementary means, such as incorporation of particulate fluorine-containing resin (Patent document 1 below) or backing with a carbon layer containing a fluorine-containing resin (paragraph [0023] of Patent document 2 shown below), but these supplementary means are accompanied with a difficulty that they inevitably lower the electricity collecting function of the gas diffuser. Alternatively, there has been made a proposal of weaving a sliver-form of carbon fiber having a fiber length of at least 25 mm, preferably 50-75 mm to provide a carbon fiber spun yarn of improved strength (Patent document 3 below). However, the thus-obtained carbon fiber spun yarn has a strength of ca. 0.08-0.09 N/tex, which is not yet satisfactory.
On the other hand, the pitch-based isotropic carbon fiber is mostly composed of short fibers, and a spun yarn obtained by further carbonization thereof has been commercialized. However, such a commercialized spun yarn is relatively thick, and a fabric obtained by weaving the spun yarn is caused to have too large a thickness and can only provide a gas diffuser of a lower performance.
Patent document 1: JP-A 2002-352807
Patent document 2: JP-A 2003-288906
Patent document 3: JP-A 53-81735